The determination of optical properties of tissues is important in many fields of medicine. Spectroscopy of tissue provides information about the absorption and scattering properties of the tissue. Such information is useful for specifying dosimetry in a laser protocol and characterization of the tissue for diagnosis. A clinical device to determine the optical properties of the tissue requires a probe that allows simple, accurate and reproducible measurements. A measurement device may comprise a topical probe for taking measurements of the skin or oral cavities, an endoscopic or a laparoscopic device for taking measurements of the gastrointestinal tract, or an interstitial needle catheter for minimally invasive interstitial measurements of the liver and brain.
Two tissues with different scattering properties and identical absorbing properties will re-emit a different number of incident photons. Consequently, to make accurate in vivo measurements of the concentration of an optically absorbing pigment, e.g., hemoglobin, it is necessary to determine both the absorption and the scattering properties. A variety of such measurements have been described.
In one example, a source fiber irradiates a sample while a detection fiber at a distance from the source fiber receives the back-scattered radiation from the sample. Multiple measurements of the back-scattered light as a function of the distance of the detection fiber from the source fiber are used to calculate the absorption and scattering properties of the sample. The distance between the two fibers typically is approximately 5-10 millimeters (mm), and may be on the order of several centimeters (cm). This application requires multiple measurements with movement of the detection fiber between each measurement.
Another application uses a single fiber for illumination and detection. This allows only a single measurement to be made and, consequently, one cannot discriminate the scattering and absorption properties but only one lumped combination of both.
Yet another application involves taking multiple measurements using the time-dependence of light that is re-emitted by the sample. This method requires high speed light detectors and complicated equipment.
Still another application uses measurements of light modulated at multiple frequencies to determine the absorption and scattering coefficients of a sample. This application requires two light sources or manipulation of the light source between measurements.